Arches are known in the construction industry. Arches span an opening and enable the support of weight above the opening. Arches are known to provide a greater weight bearing ability due to transferring of compressive only forces to the two sides of the arch. Arches are also commonly used for architectural and design purposes.
Methods of measuring and constructing arches are known. In particular a circular arch may be scribed by fixing an arch or string to a single pivot point and rotating a writing implement or scribing instrument at a free end of the arm in a semi-circular arc. This method requires that the radius of the arch be known so that the scribing instrument may be located at the appropriate distance from the pivot point.
Many construction plans do not provide the radius of the arch to be utilized in a location. Such plans often provide only the height of the arch and the width of the opening in which the arch is to be located. It is therefore required for the construction personnel to determine the appropriate arch radius. Mathematical methods are known for calculating the radius of the arch, for example by utilizing the Pythagorean Theorem which results in:R=((LL/2)2+H2)/2H 
where R is the radius of the arch, L is the chord length and H is the height of the arch above its chord.
Although the radius may be calculated using geometric relationships, tools for calculating such relationships such as charts or calculators are often not available at a construction site. In addition, errors in performing such calculations at a construction site are also possible.
What is desirable is a reliable and expedient method and apparatus for use in measuring and scribing an arch that that does not require calculating the radius of the arch using the height of the arch and the width of the opening which the arch is required to span.